1. Field
Our invention relates to the field of condensed state burners (i.e. liquid and solid-state fuel burners) of the gun type in which the fuel is dispersed or atomized in a mist or cloud of fine particles (by use of compressed air, steam, ultrasonic agitation or other mechanical means) in a high volume, low pressure stream of primary air which is blown through an aperture into a masonry-type combustion chamber. Usually the fuel is dispersed in the primary air stream and ignited before entering the chamber in a gun-type burner, so that a flame is formed outside the chamber and blown into it; but the ignition (or both the dispersal and ignition) may occur in the chamber.
Primarily, our invention is applicable to oil-fired boilers and, secondarily, to other forms of heat producing plants fired by other liquid or solid fuels or by both at once.
Our invention relates to the preheating of secondary air fed into the combustion chamber separate from the primary air. In particular, it relates to the preheating by such air by heat drawn directly from the combustion chamber--mostly by the capture and utilization of radiant energy circulating in such combustion chamber. Our suggested specific title is "Improvements in Preheating of Secondary Air from Combustion Chamber Radiation."
2. Prior Art
To the best of our knowledge, the earliest use of radiant heat from the combustion chamber to preheat secondary air in a gun-type boiler or similar gun-type heat producing plant was achieved before 1925 by checkerwork fire-brick floors with open spaces under the fire-brick floor through which the secondary air was drawn by stack suction (later aided by a blower-pressurized plenum). Earlier preheating of primary and/or secondary air had used a heat exchanger in or under the stack (or between the combustion chamber's flue gas discharge point and the entrance to the stack) to transfer heat mostly by convection from the hot flue gas to the cold incoming air. These gave a clearly understandable and easily calculable gain in heat: Essentially, all the exchanged heat was net gain--in addition, to more subtle gains from the greater combustion efficiency due to the highly early temperature of the flame. (These subtler gains probably were not then understood or observed.)
On Oct. 25, 1966, U.S. Pat. No. 3,280,770 issued to B. F. Kwiat disclosing the use of a substantially straight baffled cast-steel pipe installed on a retrofit basis through the combustion chamber's front wall in a gun-type, oil-fired boiler. As disclosed, this depended solely on stack-induced draft for its air flow. About 1968 or 1969, Weber-Bunke Lange, Inc. of 535 Bruckner Boulevard, Bronx, N.Y. 10455 began installing similar baffled straight pipe preheaters on a retrofit basis in gun-type boilers. At first, they may have relied on stack draft for their secondary air flow, but it is certain that by 1972 they were using blower pressure to improve the air flow and using silicon carbide pipes. In 1974, they printed and distributed a brochure outlining their retrofitting services.
On Aug. 4, 1970, U.S. Pat. No. 3,522,779 issued to B. F. Kwiat disclosing a sort of internal wind box preferably of heat transmitting alloy and with an annular portion positioned adjacent to the aperture to surround the primary air stream blown through the aperture into the chamber. (As disclosed the nozzle which disperses the oil extends beyond this annular portion so that this portion does not surround the flame, but precedes it.) In one form this annular portion of the wind box has about 56 small holes aimed centripetally toward the primary air stream plus about 76 small holes discharging air rearward (in the same direction the air stream is travelling). An attempt was made to physically reduce this to practice but the alloy material used was not capable of reasonable life in the severe environment which exists in a boiler's combustion chamber. No analysis of friction headlosses or heat-transfer rates was disclosed in this patent, and we feel certain no secondary-air preheating structure generally like that disclosed could achieve q values (i.e. heat flow rates) even one quarter as great as those achieved by our structure unless his blower was designed to give very high pressures, tens of times higher than those required for our improved invention. This later Kwiat patent does show that some experts were beginning to suspect, as early as 1968, that it might be desirable to release a substantial proportion of the preheated secondary air near the base of the flame. But it also confirms that no one knew a feasible way to do this. And there was not solid theoretical nor practical evidence that this would be advantageous if achieved.
On Aug. 22, 1972, U.S. Pat. No. 3,685,946 issued to Jack Adams disclosing preheating of secondary air by a straight silicon carbide pipe (with angled vanes to give a whirling motion). This patent disclosed installing the pipe through the chamber's rear wall and blowing the preheated air in counter-flow direction, against the tip of the flame to make the flame shorter and fatter and hence supposedly hotter. The patent also showed the more usual pipe-below-the-injected-flame arrangement where the general flow direction of the preheated secondary air is roughly parallel to the general direction of the flame flow but the patentee states that the whirl rotation of this secondary air is opposite to the whirl rotation of the flame which, he says, "provides for a compression of flame front 20." This Jack Adams patent shows that the combustion industry was now aware of silicon carbide's long-lived and ultraconductive properties but gives no suggestion as to how a bulky recurved assembly of SiC components could be designed to survive in a combustion chamber (or that this would be desirable).